1. Field of the Invention
The present invention relates to an oxide superconductor which is capable of trapping a high magnetic field and maintaining its performance for a long period of time without being affected by internal or external forces such as an electromagnetic force or thermal stress or by corrosive environments, and also to a process for producing said oxide superconductor.
A superconducting material, which has a high critical current density as compared with an ordinary conducting material and is capable of passing a large electric current without any loss, has become a center of attraction as a material imparted with epoch-making characteristics in the application fields of magnets and electronics. Accordingly, research and development have vigorously been carried out in recent years on its application in the field of experimental equipment for nuclear fusion, superconductive MRI for medical diagnosis, magnetic levitation trains, electric generators, energy storage units, magnetometers and the like.
2. Description of the Related Arts
By virtue of the spectacular research and development on superconductivity that were initiated in the early stage of this century, a variety of superconducting materials have come to be known. In particular, by the research and development on a metal oxide superconducting material that were initiated in the middle of 1970, there have been found metal oxide superconducting materials each having a relatively high critical temperature (T) such as LiTi.sub.2 O.sub.3, Ba(Bi,Pb) O.sub.8 and (Ba, K)BiO.sub.8. In addition, there have successively been created, from 1986 to the present time, copper oxide superconducting materials each having such a relatively high (T) that had never been anticipated before, such as (La, Sr).sub.2 CuO.sub.4, REBa.sub.2 Cu.sub.3 O.sub.7 (RE is a rare earth element), Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10, Ti.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 and HgBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.8.
As described hereinbefore, a superconducting material has a high critical current density as compared with an ordinary conducting material, and thus is capable of passing a large electric current without any loss. However, it is known that in the case of passing such a large electric current, a material is sometimes destroyed depending upon its strength, since a large electromagnetic force acts on a superconductor in question.
Accompanying the enhanced characteristics and large scale operation of relatively high temperature bulk superconductors (particularly, a copper oxide superconductor), the magnitude of a magnetic field capable of being trapped in a bulk superconductor has recently been drastically enhanced, for instance, to the extent that a magnetic flux density exceeding 5 tesla (T) has come to be trapped (refer to "Superconductor Science and Technology" 11, 1998, pp 1345 to 1347). Since an electromagnetic force applied to a superconductor increases with an increase in a trapped magnetic field, there has recently been brought about such a situation in that a restriction is imposed on a trapped magnetic field depending upon the material strength. Under such circumstances, importance is attached to an improvement in mechanical properties rather than a further improvement in a superconducting properties (refer to "Physica C" vol. 7, No. 9, 1991, pp 4989 to 4994 and "Superconductor Science and Technology" 11, 1998, pp 1345 to 1347).
It being so, the following two proposals have been made as a means for reinforcing an oxide bulk superconductor.
One proposal includes a method in which Ag is added to a material in question. It is said that by taking such measures, a remarkable improvement is brought about in the mechanical strength of an oxide bulk superconductor (refer to "Japanese Journal of Applied Physics" vol. 70, No. 9, 1991, pp 4989 to 4994).
The other proposal includes a method in which a compression strain is applied in advance to a material in question by fitting a bulk superconducting material with a metallic ring (refer to "Extended Abstract of ISTEC International Workshop" 1998, pp 115 to 118). It is said that by taking such measures, an improvement is brought about on the trapped magnetic field, since the tensile stress caused at the time of trapping the magnetic field is alleviated by the compression strain which was applied in advance, thereby suppressing the destruction of the material.
Nevertheless, the above-mentioned methods including the reinforcement with Ag addition and reinforcement with a metallic ring are desired to make further improvements in the aspects of workability and manufacturing cost. Moreover, the problem has been recognized in that the reinforced performance is deteriorated by long-term use under a corrosive environment.